1. Field of the Invention
The invention relates generally to methods and feedstock for making silica.
2. Background Art
Silica is a particularly suitable material for forming the core and cladding of an optical waveguide. For the core of the optical waveguide, silica may be doped with a small amount of other dopant materials, such as titanium oxide, tin oxide, phosphorous oxide, aluminum oxide, and germanium oxide, to slightly increase the refractive index of the core above that of the cladding. In other fibers the core is pure silica and the cladding is down doped with fluorine. Optical waveguides exhibiting very low losses are generally formed by vapor deposition processes. In one such process, a silica precursor is introduced into a conversion flame to produce fine particles, called xe2x80x9csoot.xe2x80x9d A dopant material, such as GeCl4, may also be introduced into the conversion flame along with the silica precursor (e.g., SiCl4). The soot is deposited on an outside surface of a rotating mandrel to form a soot preform. After an appropriate diameter of the core of the waveguide is reached, the mandrel is removed from the soot preform, leaving a hole in the soot preform. The soot preform is then sintered to form a consolidated glass draw preform. Next, the preform is drawn to close the hole and stretch the preform into a core cane of constant diameter. The core cane is then cut into segments, each of which becomes a deposition surface for cladding. The core cane is overclad with silica soot to an appropriate diameter and again consolidated. The resulting preform is then drawn until an optical waveguide having the desired dimensions is formed. For multi-segment fiber profiles, such as W profiles or ringed profiles, this process may include several steps of deposition, consolidation, and core-cane formation, with the dopants being provided to accomplish the desired refractive index for each segment.
Silica has also found a variety of uses in applications requiring transmission of ultraviolet radiation, particularly at wavelengths below 300 nm. One such application is in microlithography systems, which are employed in the production of integrated circuits. These systems use multiple fused silica lenses, called stepper lenses, to transmit radiation from excimer lasers to photosensitized silicon wafers. Current microlithography systems use 248-nm radiation (KrF laser) or 193-nm radiation (ArF laser) to print patterns with width as small as 0.25 xcexcm. More advanced microlithography systems using 157-nm radiation (F2 laser) are actively under development and are expected to produce patterns with even smaller widths. Microlithography systems require lenses made from high-purity fused silica because impurities in the lenses can distort the images projected onto the wafers as well as decrease transmission of the lenses. High-purity fused silica is typically produced by the boule process. The conventional boule process involves passing a silica precursor into a flame of a burner to convert the silica precursor to soot. The soot is then directed downwardly to a bait and immediately consolidated into dense, transparent, bulk glass, commonly called a boule. The boule can be used individually to fabricate stepper lenses as well as other optical elements such as photomasks.
It has been found that the selection of the silica precursor used in the production of silica is as important as the design of the equipment used to produce the silica. For a long time, the standard feedstock used in the production of silica was SiCl4. SiCl4 was chosen because it yielded large amounts of vapors at low temperatures. Flame combustion of SiCl4, however, has a drawback because it produces chlorine gas as a by-product. If the conversion flame is provided by combustion of a hydrogen-containing fuel, which is usually the case, hydrogen chloride gas is also produced as a by-product. Moreover, large amounts of water (H2, OH, and H2O) are formed. These gases are environmentally unfriendly and require considerable care for their disposal. In addition, chlorine has been found to decrease transmission at 157 nm. This makes chlorine-based silica precursors unsuitable for making, for example, fused silica lenses for 157 nm applications.
Hydrogen-containing organic compounds such as octamethlytetrasiloxane and silane have been identified as satisfactory chlorine-free precursors for producing silica. However, flame combustion of these precursors also inherently result in the silica containing residual water, i.e., OH, H2, and H2O. In optical waveguides and 157-nm applications it is preferable that the silica is substantially free of residual water. For optical waveguides, residual water in the silica results in high transmission loss in the optical waveguide. In preparation of fluorine-doped soot preform for optical waveguides, residual water is detrimental becomes it promotes fluorine migration. Water is known to reduce transmission of fused silica at wavelengths below 185 nm. Therefore, there is a strong interest in a method of producing silica that is substantially free of water and chlorine.
In one embodiment, the invention relates to a method for making silica which comprises delivering a silica precursor comprising a perfluorinated group to a conversion site and passing the silica precursor through a flame to produce silica soot. The perfluorinated group preferably comprises a group selected from the group consisting of alkyl, alkenyl, alkoxy, and aryl.
In another embodiment, the invention relates to a method for making silica which comprises a silica precursor comprising a chloro-derivative or fluoro-derivative or both to a conversion site and passing the silica precursor through a flame to produce silica soot.
Other features and advantages of the invention will be apparent from the following description and the appended claims.